1. Field of the Invention
The invention relates to the preparation and use of modified polydialkoxysiloxanes as protective barriers and as treatments for skin damage.
2. Description of Background Art
Silicone fluids having remarkable versatility and new uses are continually being developed and sought. The name “silicone” denotes a polymer with the formula shown in I:
                                          (                                                            R                  n                                ⁢                                  SiO                  4                                            -              n                        )                    m                2                    I      
where n is between 0 and 3, m is 2 or larger and R is typically alkyl. The term “silicone” arose from the work of early investigators who erroneously assumed that oxygen was bound to silicone by a double bond similar to that in ketones.
The simplest silicones are polydimethyl siloxanes having the structure:CH3—Si—O[—Si—O]n—Si—CH3  II
The repeating unit of the polymer, [—Si—O—], the dimethylsiloxane unit, in the dimethoxysiloxane unit shown in II above is referred to as the D unit so that the structure above is often referred to as MDnM. Structure II is shown as linear, although silicones can have branched points referred to as T and Q groups. Branch points form a network and introduce rigidity into the structure.
The methyl group in I can be substituted by other groups, most commonly phenyl, vinyl and hydrogen. The substitutions, branching and molecular weight of the silicone polymers determine properties and use.
Low molecular weight siloxanes are used as monomers and terminators in the polymerization of higher molecular weight silicones.
In order to provide water soluble silicones, several methods to chemically modify the silicone structure in order to circumvent the need for emulsification have been tested. Some commercially available compounds include dimethicone polyols (SILTECH), cyclomethicones (CLEARDO) and polyesters (ZENITECH).
Liquid silicone polymers, such as polydimethoxysiloxane and polydiethoxysiloxane, have a very low solubility in water or aqueous solutions. In order to accommodate this property, slightly soluble siloxane compounds can be emulsified with surfactants to form aqueous emulsions, which are stable for long periods up to several months. Such emulsions consist of a suspension of very small droplets of liquid silicone that may be visible microscopically, but are more easily characterized and monitored by light scattering. Despite the inherent instability of most emulsions, many cosmetic products have been successfully based on emulsification. Extensive experimentation in selecting the optimal surfactant and in confirming the emulsion stability of the product over an appropriate time period is usually necessary.
Efforts have also been made to develop hydrophobic silicones; for example, a reaction between alcohols and alkoxysiloxanes is typically used to introduce hydrophobic properties. Wood contains hydroxyl groups and, in the form of a finely divided flour, has been reacted with oligoethoxysiloxanes to produce a solid material with low water affinity and compatibility with hydrocarbon polymers.
Personal care products rely heavily on the use of silicones with at least half of the U.S. market for silicones used in antiperspirants and deodorants. Cosmetics traditionally use formulations that include octamethylcyclotetrasiloxane, also known as D4, in topical skin creams because of its lack of toxicity and low cost. A related compound, D5 or decamethylcyclopentasiloxane has to some extent replaced use of D4, which recently has raised concerns related to bioaccumulation, particularly in marine environments. Despite these concerns, the general consensus among manufacturers and suppliers appears to be that the cyclic silicones are safe. In a recent study, the Canadian government determined that siloxane D5 is safe for use in personal care products such as topical skin creams, antiperspirants and hair products. These siloxanes are used in amounts varying from 0.01% in skin cleansers to up to 0.65% in hair conditioners and hair grooming products. The market in 2009 for cyclic methyl siloxanes was 104 million pounds (Chem & Engineering News, vol. 89, No. 18, May 2, 2011).